From a methodological point of view the relevance of clinical-biochemical trials depends on the answers to mainly four complexes of questions: (1) the reliability of the assays in the clinical situation to be tested, (2) the precision and accuracy of sample-taking, (3) the qualification of the design and the protocols in the clinical part of the trial and (4) the usefulness of the time concepts in the trial concerning biorhythms, seasonal influences, psychological trauma of diagnostic procedures and treatment. In this study mainly the second complex of questions was studied intensely. The precision of the fluorometric histamine assay in biopsy specimens from human gastric mucosa depended on several conditions: Biochemical technique, sample preparation and removal of biopsies from gastric mucosa via endoscopy. The CV% of the whole procedure was about 8-times higher than that of the biochemical technique. In clinical-biochemical studies on the significance of histamine or any other hormone (such as gastrin) in any disease (uch as duodenal ulcer) it seems therefore useless to describe the precision of an assay only by the variance of the biochemical technique. Calculation of the histamine content as mean of 3 samples reduced the CV% from 27.2 to 14.9% and should therefore be recommended.